The invention relates to a device for distance measurement.
A device of this type is known from a publication from the company of Wild Heerbrugg AG, Switzerland, V.86, under the title "Distanzmessung nach dem Laufzeitme.beta.-verfahren mit geodatische Genauigkeit" (Distance measurement according to the propagation time measurement method with geodetic accuracy). It is also used for the measurement of distances to objects having natural rough surfaces. Thus, in the surveying of surfaces which are difficult to access such as quarries, cavern walls, tunnel profiles, etc., in which distances up to several 100 m must be measured, devices are used in which pulsed infrared semiconductor laser diodes with large emitting surfaces serve as radiation sources. Pulse lengths of 12 nsec are used. The advantage of these radiation sources consists in the fact that radiation pulses of high peak power in the order of magnitude of several watts can be generated, with the result therefore that the required measuring distances of several 100 m can be reached. The accuracy is 5-10 mm. One disadvantage results from the relatively large dimensions of the emitting surface of these lasers of the order of magnitude of 300 .mu.m. Such large dimension cause the radiation lobe from these devices to have a divergence of about 2 mrad, as a result of which, at only 50 m, a beam cross section of 0.1 m is present. In the case of a very short distance, the beam cross section of this device still has a diameter of several centimeters since in order to transmit the pulsed power of several watts at 2 mrad beam divergence, objective lens diameters of several centimeters are needed.
Since the transmitting and receiving objective lenses are arranged separately, for the near range below 10 to 15 m, an auxiliary lens must be fitted to cover the transmitting and receiving beams. A further disadvantage consists in the fact that, because of the infrared measurement radiation, the point which is currently being measured on the object is not detectable. In order to make the target visible, an additional laser with visible radiation emission is provided, the beam axis of which must be carefully adjusted to the transmitting beam axis. Such a device is equipped with an electronic evaluation and display device, which also permits additional values to be entered via a keyboard and calculations to be carried out.
Likewise, a distance measuring device with separate transmitting and receiving objective lenses is known from DE 40 02 356 C1. The transmitting device contains two electronically complementary-switchable laser diodes, one of which sends the light wave trains on the measuring path, the other sending the light wave trains on the reference path. Both light wave trains are alternately received by the same photoreceiver, which is connected to an electronics actuation means. Whether the laser diodes emit visible light cannot be gleaned from the publication. The distance range to be measured is specified as 2 to 10 m and the measurement accuracy is intended to be in the range of a few mm.
In the journal "Industrie", Nov. 1992, pages 6-8, a distance measuring device DME 2000 from the Sick GmbH company is described, the device carries out an optical distance measurement on the basis of propagation time measurement, and operates with two semiconductor laser diodes emitting visible light. The required transmitting light is generated by a laser diode with collimator optics and, the second laser diode supplies the necessary reference signal directly to the receiver. The transmitting beam and the receiving beam are arranged to be coaxial to each other, so that only one single objective lens of relatively large diameter is used. The measuring distance to natural rough surfaces is 0.1 to 2 m with a light spot diameter of about 3 mm. For greater distances from the object, up to 130 m, a reflector film must be applied to the object to be measured. The light spot diameter at these distances is about 250 mm. In conjunction with the coaxial transmitting-receiving optics, a relatively large-area PIN photodiode is used as receiver. Hence, although an overlap of the strongly divergent receiving light cone with the transmitting beam is achieved, with the result that distances down to 0.1 m can be measured, no great measurement ranges can be achieved with these large-area detectors without additional reflectors.
In the construction industry, in particular in internal construction and in the installation industry, it is necessary to be able to measure distances of up to 30 m on rough surfaces without the additional reflectors. In the case of a required measurement accuracy of 1 to 2 mm, the divergence of the receiving beam must be as small as possible, since otherwise the ambient light proportion received at the same time would generate too large a noise signal in the receiver. A small divergence of the receiving beam of about 2 mrad, however, has the disadvantage that, in the case of separate transmitting and receiving optics, an overlap of the receiving beam with the transmitting beam is only present after 1 to 2 m, so that only distance measurements beyond this distance are possible.